Cool drying is, as is known, based on the principle that by lowering the gas temperature the moisture in the gas condenses, after which the condensate is separated in a liquid separator and after which the gas is again heated such that this gas is no longer saturated.
It is known that in most cases compressed air, supplied by a compressor for example, is saturated with water vapour or, in other words, it has a relative humidity of 100%. This means that in the event of a temperature drop to below the ‘dew point’ condensation occurs. Because of the condensed water corrosion occurs in the pipes and tools that draw off compressed air from the compressor, and equipment can present premature wear.
It is consequently necessary to dry this compressed air, which can be done in the aforementioned way by cool drying. Air other than compressed air or other gases can also be dried in this way.
When drying compressed air, the air in the heat exchanger cannot be cooled too much as otherwise the condensate could freeze. Typically the dried compressed air has a temperature equal to two to three degrees above zero or 20° C. below ambient temperature. The coolant temperature in the evaporator is kept at between 15° C. and −5° C. for this purpose.
To prevent the condensate from freezing, as is known, the compressor speed is controlled as a function of the measured lowest gas temperature LAT. The LAT is the lowest occurring temperature of the gas to be dried that is guided through the secondary section of the aforementioned heat exchanger.
If the LAT decreases and the condensate threatens to freeze, for example due to the gas flow decreasing, the compressor speed is reduced such that the LAT increases again.
If the LAT increases, for example due to the gas flow increasing, the compressor speed is increased such that the evaporator temperature falls and the LAT will also fall.
A disadvantage of control on the basis of the LAT is that the evaporator temperature can become too low, such that freezing can occur in the evaporator.
Control on the basis of the evaporator pressure, in other words the pressure in the evaporator, is also known. In such a case the compressor speed is controlled such that the evaporator pressure is maintained between certain limits.
A disadvantage of the aforementioned control is that in the event of a low load of the cooling circuit, or for example with a low supplied gas flow, the condensate can freeze.
Another additional disadvantage of control by means of controlling the speed of the compressor is that a compressor always has to be used whose speed can be adjusted.
Moreover, the speed of such a compressor must always be kept within certain limits, such that in certain cases freezing of the condensate cannot be prevented.
To ensure that the air in the heat exchanger does not cool too much, for example in the event of a variable load of the cool dryer, another already known approach consists of keeping the coolant temperature under control by providing the device with at least one bypass pipe across the compressor. A mechanical control valve in an aforementioned bypass pipe enables a certain quantity of coolant, in the form of hot gas, to be tapped off from the cooling circuit if need be, and to then be driven through the aforementioned bypass pipe across the compressor. In this way the cooling capacity of the device can be reduced and can prevent the condensate from freezing in the heat exchanger or the temperature of the coolant falling too greatly.
The mechanical control valve is hereby controlled by a control unit that is connected in a known way to one or more sensors, whereby these sensors determine the LAT.
When the aforementioned sensors register a lowest gas temperature (LAT), whereby freezing of the condensate can occur, the control unit sends a signal to a mechanical control valve to open the latter. In this way a certain quantity of coolant is guided across the compressor via an aforementioned bypass pipe so that the cooling capacity of the cooling circuit is decreased.
If the lowest gas temperature (LAT) is more than two to three degrees above zero, the mechanical control valve is closed so that the entire capacity of the cooling circuit is utilised to sufficiently cool the gas to be dried.
However, such known installations also present the disadvantage that the mechanical control valve can only be set to a completely open or completely closed state.
Consequently the supply of coolant and consequently the decrease of cooling capacity cannot be adjusted to the specific situation of the time or to the load at that moment.
A disadvantage of this is that it is possible for the coolant temperature to be increased too much such that the cooling capacity decreases too much and that the mechanical control valve is repeatedly opened and closed.
Moreover, due to the use of a mechanical control valve large fluctuations occur in the temperature of the coolant such that fluctuations occur in the dew point or the lowest gas temperature.
In the known method for cool drying, the aforementioned expansion means are controlled on the basis of measurements of the evaporator pressure and evaporator temperature.
As is known, the function of the expansion means consists of expanding just enough coolant so that the coolant always enters the cooling compressor with the desired degree of superheating.
Due to this superheating the liquid coolant present can be evaporated before being guided to the cooling compressor in order to give the cooling compressor optimum protection against liquid coolant.
The superheating of the coolant can be determined on the basis of the measurements of the evaporator pressure and the evaporator temperature, and it can be determined whether the expansion valve has to be opened more or less in order to be able to control the superheating of the coolant.
In order to make an accurate calculation of the superheating, both measurements must be done at exactly the same location. In this way a pressure loss in the cooling circuit and/or the bends of the cooling circuit has no effect on the pressure measurement.
The evaporator temperature is measured on the outside of the cooling circuit in a known way.
Such known installations thus present the disadvantage that the measurement is very slow and lags a possible change in the evaporator temperature.
This has the disadvantage that the calculation of the superheating is also slow and is not accurate as a change in the superheating is not immediately detected. As a result the expansion valve is not well controlled and not controlled quickly enough to control the superheating of the coolant.